This invention relates to an improved process for the preparation of pentaacetyl-xcex2-D-glucopyranose useful as intermediates for the synthesis of raw materials for industrial chemicals, medicines and cosmetics, for example, Arbutin or its related compounds, and the like. More particularly, the invention relates to a process of preparing pentaacetyl-xcex2-D-glucopyranose of high purity in high yield.
A process for the preparation of pentaacetyl-xcex2-D-glucopyranose is described by M. L. Wolfrom and A. Thompson in xe2x80x9cMethods in Carbohydrate Chemistry, vol. 2, Academic Press, pp. 211-212 (1963)xe2x80x9d. This method performs an acetylation of D-glucose with acetic anhydride at high temperature in the presence of sodium acetate catalyst.
In the method described in the above-mentioned literature, acetic anhydride is used in an amount of 13 mols per mol of D-glucose, that is, in an excess amount corresponding to 2.6 times a theoretical amount. However, a problem arises that the reaction of such an excess amount of acetic anhydride with D-glucose at a high temperature (e.g., 100xc2x0 C.) in the presence of sodium acetate catalyst will bring about a rapid progress of reaction, leading to the difficulty in control. In this method, it is assumed that an addition of acetic anhydride in more than two times amount to five reaction sites (hydroxyl groups) in D-glucose will result in rapid progress of reaction.
For the preparation of pentaacetyl-xcex2-D-glucopyranose, the above reference method has performed the acetylation of D-glucose as follows:
xe2x80x9c50 g of anhydrous sodium acetate and 700 ml of acetic anhydride are placed in a 2-liter, round-bottomed flask, and heated over a flame to the boiling point in a hood. About 3 g of anhydrous D-glucose from a 100 g supply is added, the flask, without shaking, is heated carefully at the point nearest the sugar laying on the bottom. Initiation of the reaction is indicated by continued boiling after removal of the flame. Then, the flame is extinguished and the flask is placed on a cork ring. The remainder of the sugar is added in small portions at a rate which maintains the boiling temperature of the mixture. The flask is occasionally shaken to prevent an accumulation of solid sugar on the bottom of the flask. If the reaction stops, it should be started again by heating before much more sugar is added to the flask. After the addition of all the sugar and after the reaction has subsided, the solution is brought to a full boil.xe2x80x9d (See p. 212 of the same literature.)
Thus, the above reference method has taken a measure of conducting careful reaction, while adding D-glucose in small portions, to prevent a rapid progress of reaction.
Such a series of complicated steps makes an industrial production very difficult. Further, the above-mentioned literature describes in the middle of page 212 as follows: xe2x80x9cThe reaction solution is cooled and poured with stirring onto 2 liters of cracked ice. After standing 3 hours with occasional stirring, the crystalline material is filtered and washed with cold water. Yield 160 g (73%).xe2x80x9d The yield is not satisfactory. About the purification of the crude product, there is the following description:
xe2x80x9cPurification is effected by recrystallization from 1 liter of hot 95% ethanol followed by filtration with decolorizing carbon. The product is filtered as soon as the temperature of the crystalline material has cooled to room temperature. Further recrystallization in the same manner produces pure pentaacetyl-xcex2-D-glucopyranose.xe2x80x9d However, this method requires a treatment with decolorizing carbon or the like, and takes a lot of time and labor in the purification. In addition, the method requires twice recrystallization for the preparation of pure pentaacetyl-xcex2-D-glucopyranose. Though there is no reference to a yield of the purified product, it is presumed to be about 50-60%. Therefore, there is a demand for a process for the preparation of pentaacetyl-xcex2-D-glucopyranose in a satisfactory yield under industrially practicable reaction conditions.
As a result of extensive investigation in an effort to prepare pentaacetyl-xcex2-D-glucopyranose efficiently, the present inventors have found that pentaacetyl-xcex2-D-glucopyranose of high-purity can be prepared in high yield under very mild reaction conditions by using acetic anhydride of 1-2 times the theoretical amount relative to D-glucose and performing the reaction in an organic solvent in the preparation of pentaacetyl-xcex2-D-glucopyranose by reacting acetic anhydride with D-glucose in the presence of sodium acetate catalyst.
Thus the present invention provides a process for the preparation of pentaacetyl-xcex2-D-glucopyranose by reacting D-glucose with acetic anhydride in the presence of sodium acetate catalyst, characterized in that 5-10 mols of acetic anhydride are used per mol of D-glucose and the reaction is performed in an organic solvent.
An amount of acetic anhydride used in the invention is theoretically enough in one mol per mol of a hydroxyl group in D-glucose, but up to 2 mols will not be an obstacle to the control of reaction. However, more than 2 mols are not preferable, since the reaction proceeds with violence. Less than one mol ratio is not preferable, since an acetylation of D-glucose does not proceed completely, leaving hydroxyl group(s) unreacted, which results in lowering a yield of a desired end product.
The solvents used in the invention can include aliphatic hydrocarbons such as pentane, hexane, heptane, petroleum ether and the like; aromatic hydrocarbons such as benzene, toluene, xylene and the like; acetic acid esters such as ethyl acetate, propyl acetate, isopropyl acetate, isobutyl acetate, butyl acetate and the like; chlorinated hydrocarbons such as chloroform, carbon tetrachloride, 1,2-dichloroethane and the like. They are preferably used alone, but may be used in combination with some kinds of organic solvents. The organic solvents are used in the amount of 5-20 times by volume, preferably 5-10 times by volume, per part by weight of D-glucose. For example, the amount of 5 times by volume of the organic solvent per part by weight of D-glucose means 150 ml of the organic solvent relative to 30 g of D-glucose.
The sodium acetate catalyst can be used in the amount of 0.1-1 mol, preferably 0.1-0.5 mol, per mol of D-glucose.
Usually, the reaction is carried out by heating a mixture of D-glucose, acetic anhydride and sodium acetate in the organic solvent. This reaction is preferably carried out at boiling point of the reaction solvent used, i.e., under reflux. The reaction time ranges from 30 minutes to 24 hours, depending on kinds of the reaction solvents used and other factors.
After completion of the reaction, water is added to the reaction mixture to decompose unreacted acetic anhydride. The reaction mixture is neutralized with an aqueous alkali solution, and the organic layer is separated. The organic layer is washed with water, if necessary.
The solvent is evaporated off from the organic layer and the resultant crude crystals of pentaacetyl-xcex2-D-glucopyranose are crystallized in a solvent for recrystallization such as ethanol, or alternatively a part of the organic solvent is distilled off from the organic layer and the desired pentaacetyl-xcex2-D-glucopyranose is crystallized from the remaining solution, by which a purified product can be obtained.
Thus, according to the invention, pentaacetyl-xcex2-D-glucopyranose of high-purity can be prepared in high yield by an industrially practicable method.